"That day, when I was asked to do something I really didn't want to do, something I had put off for more than a year, I had no way of knowing that I was in a life-or-death situation," she wrote in a blog post Monday. "Sitting in that kitchen with Marie Monville, I had cancer and didn't know it. In fact, I would have considered it virtually impossible that I would have cancer. I work out, I eat right, I take care of myself and I have very little family history; in fact, all of my grandparents are still alive."

Robach says she plans to have the mastectomy on Nov. 14. "Only then will I know more about what that fight will fully entail, but I am mentally and physically as prepared as anyone can be in this situation," she writes. "And while everyone who gets cancer is clearly unlucky, I got lucky by catching it early, and there are so many people to thank for making sure I did. Every producer, every person who urged me to do this, changed my trajectory."